INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Fixed Maturities, by Expected Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale, Amortized Cost
Amortized Cost	$ 52,278,016	$ 52,452,334
Available-for-sale, Fair Value
Total	51,688,233	49,898,408
Held-to-maturity, Fair Value
Fair Value	2,594,441	2,547,210
Fixed maturities
Available-for-sale, Amortized Cost
Due in one year or less	1,259,571
Due after one year through five years	8,936,843
Due after five years through ten years	8,822,759
Due after ten years	32,651,826
Amortized Cost	51,670,999	51,816,959
Available-for-sale, Fair Value
Due in one year or less	1,257,644
Due after one year through five years	8,920,426
Due after five years through ten years	8,877,578
Due after ten years	32,025,568
Total	51,081,216	49,263,033
Held-to-maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	2,607,356
Amortized Cost	2,607,356	2,633,474
Held-to-maturity, Fair Value
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	2,594,441
Fair Value	$ 2,594,441	$ 2,547,210